A wide variety of medical procedures utilize an intravascular line to infuse or withdraw blood or fluid from the human body. For example, infusion pumps are commonly used for the controlled infusion of fluid or medication through an intravascular line into a patient's bloodstream.
Many intravascular procedures relate to the extracorporeal treatment of blood, such as blood oxygenation, blood component separation and blood cleansing. Some of these procedures include rather extensive manipulation of the blood and its components. For example, hemodialysis is typically performed by withdrawing the blood via a tube that taps the patient's vascular system, transporting the blood through the tube to a dialyzer, purifying the blood with the dialyzer, transporting the treated blood through another tube from the dialyzer back to the patient, and replacing the treated blood back into the vascular system of the patient. This tube or blood set may also provide for other necessary functions such as pressure monitoring, temperature sensoring, flow detection and blood pumping. Each of these functions may require one or more blood line features such as valves or other fittings.
These procedures can introduce air into the blood line in a variety of ways. One of the most dangerous ways is by a leak in the tubing or a fitting connection. This can introduce a large volume of air which can pass into the bloodstream and cause an embolism or other life-threatening condition.
Many hemodialysis and other intravascular techniques use a device to detect air in the venous tubing before it flows into the patient. Such devices generally rely on the measurement and processing of variations in the optical or ultrasonic characteristics of the fluid in the tubing. The optical devices include a light element that shines light into the fluid flow through the transparent tubing wall. A photocell on the other side of the tubing measures the transmitted light to determine the transmissibility of the fluid, or a photocell on the same side of the tubing measures the reflected light to determine the reflectivity of the fluid. The system can be calibrated to detect and identify predictable transmissibility or reflectivity variances caused by air bubbles.
In the case of ultrasonic devices, which are among the most prevalent of air detection devices used with hemodialysis machines, a segment of the blood set is clamped between ultrasonic transducers that send ultrasonic signals through the blood set wall, through the flowing fluid, and through the opposite blood set wall. The blood without air bubbles has a fairly consistent and predictable transmissibility to the ultrasonic signal. Therefore, any variations in the received signal caused by air bubbles (or even foreign particulates, for that matter) can be detected, processed and measured against the normal signal. Variances exceeding a predetermined absolute threshold, or a predetermined cumulative threshold over a period of time, are deemed to represent an unacceptably high air level. The transducer is coupled with an alarm to warn the operator of the high air level and is also electronically connected to a clamp or other device to occlude the tube and stop the blood pump to prevent the detected air from flowing into the patient.
Due to a variety of factors, the sensitivity of existing air bubble detectors is not always as high as desired. Therefore, the alarm thresholds are typically set conservatively low. This protects the patient but also causes a high number of false alarms. The prior art approach to improving the sensitivity of air bubble detectors has relied largely on making modifications to the electronic circuitry that analyzes the received signals. See, for example, the systems that accumulate signal variances over a period of time and compare the accumulated variances against predetermined thresholds, that are described in U.S. Pat. Nos. 4,341,116 by Bilstad, 3,935,876 by Massie, 4,651,555 by Dam, and 4,487,601 by Lindeman.
One drawback to the approach of increasing the sensitivity of the detector by modifying the electronic circuitry that processes the received signal, is that this approach requires replacing or modifying the circuitry on existing machines that are in use.
Accordingly, there is a need for an approach to increasing the sensitivity of an air bubble detector without modifying the transducers or electronic circuitry of the existing devices. Preferably, the modification would be inexpensive and effective, and a given modification could be interchangeable with a variety of other modified or unmodified versions depending on the sensitivity desired in particular applications.